Indebtedness (Unsecured Line of Credit) (Details) (Unsecured Line of Credit DRLP [Member], USD $)	3 Months Ended
	Mar. 31, 2015
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,600,000,000
Line of Credit Facility, Covenant Compliance	we were in compliance with all covenants under this line of credit.
Current Market Rate Of Unsecured Lines Of Credit	1.43%
Line of Credit Facility Option to Increase Borrowing Limit	400,000,000
Unsecured Line of Credit DRLP [Member]
Maximum Capacity	1,200,000,000
Maturity date	Jan. 01, 2019
Line of credit balance	$ 453,000,000
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	1.05%
Line of Credit Facility, Interest Rate at Period End	1.23%